The field of communications has benefited enormously from the introduction of optical communications technology. Fundamentally, this technology exploits the inherent bandwidth potential of the light itself as a carrier. As this technology natures, the need for the direct optical processing of signals is becoming greater. Much of the communications infrastructure in operation in the field relies on optical signals being converted back to electrical form for certain processing and management functions. Direct optical processing has the benefit of avoiding the need for such conversion with its associated costs, losses and amplification requirements.
One of the fundamental building blocks of an optical communications system is the optical cross-connect or optical crossbar switch. These devices function to selectably connect any one of an array of incoming optical signals to any one of an array of outgoing channels. Inherently these devices consist of a multiplicity of optical communications channels that are often implemented on one semiconductor device wafer using micro-machining technology.
A variety of specific structures for optical crossbar switches have been proposed. While many of these rely on non-linear optic materials to obtain switching actions, a very popular way to achieve this end at the time of this application for letters patent is by means of micro-electromechanical structures. These are usually micro-mirror devices that tilt, flex, or flip upon application of an appropriate control voltage.
Most typically, these devices have two states, one of which causes an incoming beam of light to bypass the mirror, by flipping the mirror down or out of the way, and a second position in which the mirror is interposed in the path of the beam so as to reflect it into some or other desired direction in order to couple the optical beam into an output channel, usually via a micro-lens and optical fiber arrangement.
Since one of the very strengths of optical communications is the very wide bandwidth that it makes possible, there is every incentive to ensure that the switching devices are commensurately fast, as this determines the rate at which routing and managed networking of the communication may be achieved.
At the device levels this creates a desire for the reflective elements to have the highest possible natural resonant frequency. While materials choice for the reflective element can help to make this frequency as high as possible, the very size of the mirror structure is core issue. The reflective element needs to be as small as possible.
This requirement presents a problem in that the small apertures involved in the cores of the optical fibers carrying light signals lead to considerable beam divergence, which is typically addressed via micro-lenses to collimate the emerging beam. However, this collimation is also inherently limited by the aforementioned aperture dimensions with the result that it is very difficult to maintain very narrow beam widths across the lateral extent of a multi-channel crossbar switch. The mirrors therefore have to be larger than the beam width in order to reflect most of the incident beam. This requirement for larger mirrors is contrary to the need for high speed switching.